1. Field of the Invention
The invention relates to a shutter brake mechanism for use in a photographic device, such as a camera. The shutter brake mechanism can prevent damage to shutter blades and improve the durability of the shutter, shutter blades and brake mechanism.
2. Description of Related Art
A known shutter blade travel motion brake mechanism in a camera is disclosed in Japanese Laid-Open Patent Publication Sho JP 61-251830. In JP 61-251830, FIG. 7, a drive transfers a friction torque to a brake lever. The friction torque is created by the lever being pressed against a washer by a disc spring. A driving pin then contacts a brake lever near the completion of a blade travel motion mechanism's action. A braking action is applied to the blade travel motion mechanism through the friction torque. In conventional devices, a surface hardness of the brake lever is Hv 450 to 550, and the surface hardness of the driving pin is Hv 700-800, where the surface hardness is a Vicker's hardness (Hv), and represents the value for a 100 g load. Hereafter, all surface hardnesses will be established according to the Vicker's hardness.
Recently, the need to increase a camera's speed, by boosting the speed and maximum time of the shutter, has grown. Consequently, it has become necessary to increase the blade travel motion mechanism's speed. However, by increasing the speed, the shutter blades incurs increased damage. The degree of damage to the shutter blades depends on the strength of the blades themselves. Thus, it is necessary to strengthen the brake mechanism with an increased brake speed to prevent damage.
In conventional devices, when the spring force is increased, the created friction torque is also enlarged. A driving pin makes contact with the brake lever near the completion of the blade travel motion mechanism's motion. The driving pin collides with the brake lever at high speed. Accordingly, when the friction torque of the brake lever is increased without other modifications, the driving pin and brake lever are damaged. Thus, the performance of the brake mechanism decreases and the durability of the shutter is decreased.
The above-described damage will not occur at a blade travel motion time with a 2.9 ms shutter blind speed. However, damage of brake lever and driving pin occurs from a small number of shutter actions, when the shutter blind speed is 2.5 ms. When the shutter blind speed is faster than 2.2 ms, the damage was extreme and the shutter blades were repeatedly damaged. The shutter blind speed is defined as the time required for the slit edge of the shutter blades to pass through the picture plane.
When, as in conventional devices, the surface hardness of the brake lever is less than the surface hardness of the driving pin, the damage to the brake lever is extreme. As a result of the increased impact force, damage to the shutter blades will also increases.
The degree of damage depends on the thicknesses and surface hardnesses of the brake lever and the driving pin. It is possible to decrease the damage to the brake lever and the driving pin, while increasing the contact surface area between the brake lever and the driving pin, if the thickness of the brake lever is increased. Additionally, by increasing the surface hardness of the brake lever and the driving pin, it is also possible to reduce damage thereto. However, increasing the thickness of the brake lever causes the shutter to have a larger size. When the surface hardness brake lever and of the driving pin are increased, the ability to withstand impact decreases. Further, the brake lever or driving pin may crack because the driving pin collides with the brake lever at high speeds.